Optical polarizing film is widely used for glare reduction and for increasing optical contrast in such products as sunglasses and Liquid Crystal Displays (LCD). One of the most commonly used types of polarizers for these applications is a dichroic polarizer which absorbs light of one polarization and transmits light of the other polarization. One type of dichroic polarizer is made by incorporating a dye into a polymer matrix which is stretched in at least one direction. Dichroic polarizers may also be made by uniaxially stretching a polymer matrix and staining the matrix with a dichroic dye. Alternatively, a polymer matrix may be stained with an oriented dichroic dye. Dichroic dyes include anthraquinone and azo dyes, as well as iodine. Many commercial dichroic polarizers use polyvinyl alcohol as the polymer matrix for the dye.
Another type of polarizer is a reflective polarizer which reflects light of one polarization and transmits light of another orthogonal polarization. One type of reflective polarizer is made by forming a stack of alternating sets of polymer layers, one of the sets being birefringent to form reflective interfaces in the stack. Typically, the indices of refraction of the layers in the two sets are approximately equal in one direction so that light polarized in a plane parallel to that direction is transmitted. The indices of refraction are typically different in a second, orthogonal direction so that light polarized in a plane parallel to the orthogonal direction is reflected.
One measure of performance for polarizers is the extinction ratio. The extinction ratio is the ratio of a) light transmitted by the polarizer in a preferentially transmitted polarization state to b) light transmitted in an orthogonal polarization state. These two orthogonal states are often related to the two linear polarizations of light. However, other types of orthogonal states, such as, left and right-handed circular polarizations or two orthogonal elliptical polarizations may also be used. The extinction ratios of dichroic polarizers vary over a wide range depending on their specific construction and target application. For example, dichroic polarizers may have extinction ratios between 5:1 and 3000:1. Dichroic polarizers used in display systems typically have extinction ratios which are preferably greater than 100:1 and even more preferably greater than 500:1.
Dichroic polarizers typically absorb light in the non-transmission polarization. However, dichroic polarizers also absorb some of the light having the high transmission polarization. The amount of this absorption depends on the details of the construction of the polarizer and the designed extinction ratio. For high performance display polarizers, such as those used in LCDs, this absorption loss is typically between about 5 and 15%. The reflectivity of these polarizers for light having the absorption (i.e., low transmission) polarization tends to be small. Even with surface reflections included, this reflectivity is typically less than 10% and usually less than 5%.
Reflective polarizers typically reflect light having one polarization and transmit light having an orthogonal polarization. Reflective polarizers often have incomplete reflectivity of the high extinction polarization over a wavelength region of interest. Typically, the reflectivity is greater than 50% and is often greater than 90% or 95%. A reflective polarizer will also typically have some absorption of light having the high transmission polarization. Typically, this absorption is less than about 5 to 15%.